As illustrated in FIG. 1, an optical fiber 1 having an inclined end face 1a not perpendicular to an optical axis S is known. Output light B from the inclined end face 1a is emitted at an exit angle θ with respect to the optical axis S.
As illustrated in FIG. 2, for example, if optical fibers 1 are coupled to each other by an optical switch, since light reflected at an inclined end face is not returned, an optical fiber having such an inclined end face is advantageous in that the optical signal is less likely to be affected by noise. In this case, an alignment operation is performed to minimize coupling losses of light. The alignment operation involves moving an optical fiber in an optical axis direction x and rotating the optical fiber in an r direction about the optical axis. To facilitate the alignment operation, it is required to reduce variations in the exit angle of the optical fiber (to ±0.15° or less).
Therefore, it is necessary to perform an inspection to see if an exit angle of a manufactured optical fiber is within a predetermined range.
A method and apparatus for inspecting an exit angle of an optical fiber are disclosed, for example, in Patent Literature (PTL) 1.
In this method and apparatus, a measurement end of an optical fiber is passed through a hole formed in a holder, having an inside diameter (Dw) at least 10 μm larger than an outside diameter (Df) of the optical fiber, and having a length of at least(Dw-Df)/tan(0.1°) in the axial direction. The optical fiber is rotated using the hole as a guide hole. Then, the size of at least part of a circle or ellipse drawn on a screen by output laser light from the measurement end face is measured. Thus, an exit angle (θ4) of the output laser light from the measurement end face is measured.
PTL 1 describes the fact that unless the inside diameter of the hole is at least 10 μm larger than the outside diameter of the optical fiber, the optical fiber is not properly inserted into the hole of the holder and is bent (see paragraph [0034]).
PTL 1 also describes the fact that with this method and apparatus, the repeatability of measurements of the exit angle (θ4) is within 0.1° (see paragraph [0043]).
To reduce variations in the exit angle of a manufactured optical fiber to ±0.15° or less, an acceptance level (threshold value) used in the inspecting method of PTL 1 needs to be a predetermined angle ±0.05°. This is because the repeatability of this method is 0.1° or less. This acceptance level, which is the predetermined angle ±0.05°, is very difficult to achieve and the passing rate is very low. As a consequence, a substantial number of acceptable products are included in products which have been rejected in practice.
Therefore, it is not practical to use the exit angle measuring method and apparatus with which a measurement repeatability of within 0.1° is achieved.
As a solution to this, a holder 7 illustrated in FIG. 16 is provided.
In the holder 7, a bare portion 1c obtained by removing a sheath at an end of the optical fiber 1 is placed in a V-shaped groove 70a formed in an upper surface of a plate-like main body 70. A retaining plate 71 of the holder 7 is placed on the bare portion 1c and pressed with a predetermined force F to fix the optical fiber.
In this fixed state, output light is received by light receiving means, such as a microscope, to measure the coordinates of the light receiving position. Then, after the retaining plate 71 is removed to rotate the optical fiber by a predetermined angle, the optical fiber is fixed by the retaining plate again to measure the light receiving position of the output light. By repeating this process several times (about six times), the coordinates of several points on a locus circle of output light are determined. From the coordinates, the diameter of the locus circle is calculated to determine the exit angle.
However, due to adhesion of fine airborne dust particles to the groove 70a and the retaining plate 71, a slight distortion in circular cross section of the optical fiber, or various other causes, the locus of the output light does not draw an accurate circle or the level of variations in values obtained in repeated measurements of the same optical fiber (repeatability of measurements) is not satisfactory. Additionally, measured values tend to vary due to individual differences among persons who perform the measurements.
Citation List
Patent Literature
PTL 1: Japanese Unexamined Patent Application Publication No. 2005-233875
As described above, using the exit angle measuring method and apparatus disclosed in PTL 1 in current practice is not practical in terms of accuracy.
With the measuring apparatus using the holder illustrated in FIG. 16, the standard deviation in repeated measurements is about 1.1°×10−2, which is insufficient in accuracy. Moreover, the measuring operation is very cumbersome in that it involves rotating the optical fiber while repeatedly fixing and releasing the optical fiber to and from the holder.
An object of the present invention is to improve repeatability, facilitate a measuring operation, and accurately measure exit angles of many optical fibers in a short time.